1. Field of the Invention
This invention relates to delayed coking of heavy hydrocarbons, and more particularly to improved apparatus for improving the product yield structure from a delayed coking unit.
Delayed coking has been practiced for many years. The process broadly involves thermal decomposition of heavy liquid hydrocarbons to produce gas, liquid streams of various boiling ranges, and coke.
Coking of resids from heavy, sour (high sulfur) crude oils is carried out primarily as a means of disposing of low value resids by converting part of the resids to more valuable liquid and gas products. The resulting coke is generally treated as a low value by-product.
In the production of fuel grade delayed coke, and even to some extent in the production of anode or aluminum grade delayed coke, it is desirable to minimize the coke yield, and to maximize the liquids yield, as the liquids are more valuable than the coke.
The use of heavy crude oils having high metals and sulfur content is increasing in many refineries, and delayed coking operations are of increasing importance to refiners. The increasing concern for minimizing air pollution is a further incentive for treating resids in a delayed coker, as the coker produces gases and liquids having sulfur in a form that can be relatively easily removed.
In delayed coking process for making fuel grade coke or aluminum or anode grade coke, the coke is generally the least valuable product, and it is desired to improve the product yield structure by minimizing or decreasing the coke yield while increasing the yield of the more valuable liquid products.
2. The Prior Art
In the basic delayed coking process as practiced today, fresh feedstock is introduced into the lower part of a coker fractionator and the fractionator bottoms including heavy recycle material and fresh feedstock are heated to coking temperature in a coker furnace. The hot feed then goes to a coke drum maintained at coking conditions of temperature and pressure where the feed decomposes or cracks to form coke and volatile components. The volatile components are removed as coke drum vapor and returned to the fractionator. The heaviest components of the coke drum vapors are condensed by one of several methods, including direct or indirect heat exchange. Typically, heavy coker gas oil from the coker fractionator is cooled by heat exchange with fresh feed and then returned to the fractionator where it is sprayed into the fractionator flash zone to contact incoming vapors and condense the heavier components thereof. The heaviest components of the coke drum vapors could be condensed by other techniques, such as indirect heat exchange, but in commercial operations it is common to contact the incoming vapors with a heavy coker gas oil in the coker fractionator.
The delayed coking process is described in more detail by Kash et al entitled "Delayed Coking," The Oil and Gas Journal, Jan. 2, 1956, pp 89-90.
A delayed coking process for coal tar pitches illustrating use of heavy recycle is shown in U.S. Pat. No. 3,563,884 to Bloomer et al.
A delayed coking process for coal extract using a separate surge tank for the feed to the coker furnace is shown in U.S. Pat. No. 3,379,638 to Bloomer et al.
A process for producing a soft synthetic coal having a volatile matter content of more than 20 percent by weight is described in U.S. Pat. No. 4,036,736 to Ozaki et al. In that reference, a diluent gas is added to th coke drum to maintain a reduced partial pressure of cracked hydrocarbons, or the process is carried out under less than atmospheric pressure.
In commonly assigned co-pending Application Ser. No. 590,607 now U.S. Pat. No. 4,518,487 filed Mar. 19, 1984 by Harlan G. Graf et al for "Process for Improving Product Yields from Delayed Coking", a delayed coking process is described in which condensed coke drum vapors are removed from the process rather than being utilized as recycle. This results in a decreased coke yield and increased liquids yields. The present invention is directed to improved apparatus for carrying out such a process.